This invention relates to a sailboat having a multiple hull structure; and more particularly to such sailboat which is light in weight and which can safely carry a large sail in relation to its weight.
For many years boat designers have been convinced that a boat's speed was limited by its waterline length. The accepted formula stated: the hull speed in knots is 1.4 times the square root of the waterline length in feet. At this hull speed, the bow wave gets in phase with the stern wave, with the result that the boat is constantly trying to climb over a mountain of water of its own making.
If there is sufficient thrust available, a boat can actually climb over this mountain of water; and then, in the planing mode, it can go many times faster than the hull speed. In the planing mode, the weight of the boat is supported by dynamic forces rather than buoyant forces. However, because the drag increases with the square of the speed, planing generally requires for more power than is available on sailboats.
The reversal of the slope of the drag-versus-speed curve is called the "hump", and has been verified many times by the present inventor and others by tow testing of various hulls. The effect is most pronounced with hulls having a length only two or three times their width; and it almost disappears entirely when the hull lengths are six or seven times their width. A boat with a length-to-beam ratio of seven or more displays a smooth drag-versus-speed curve, with the drag being proportional to the square of the speed. The drag of such a boat hull is much less than the drag of a shorter hull having equal displacement. Such a hull has very little roll stability to keep the boat erect against the force of the wind, and hence the sail area which may be carried by such a boat is minimized.
One obvious answer to improving sail carrying ability is multiple-hull boats. Multiple-hull boats are able to carry more sail area per unit of weight than a single-hull boat. The best multi-hull boats are about twice as fast as single-hull boats of equivalent displacement in the same wind.
Edmond Bruce, an early contributor to sailboat racing literature, has suggested a measure of quality for sailboats called the "Bruce Number". It is the square root of the sail area in square feet, divided by the cube root of the displacement in pounds. Most single-hull boats operate with Bruce numbers in the range of 0.8 to 1.0, while the fastest multiple-hull boats operate in the range of 1.6 to 1.8.
Multiple-hull boats (catamarans and trimarans) suffer from major disadvantages which render them unseaworthy in the minds of conventional yachtsmen. Most importantly, when they capsize they will not right themselves. Secondly, the hulls are rigidly connected and this results in structurally destructive forces on the connecting members when the hulls, moving through rough water, encounter slightly different wave patterns. When the connecting beams are made stronger and larger, they produce an effective wing or airfoil which can be caught by the wind when the boat is heeling to contribute to capsizing the boat.
An object of this invention is to provide a fast lightweight multiple-hull sailboat having the ability to carry a sail of larger area than known multiple-hull sailboats.
Another object of this invention is to provide a sailboat designed so that it will not capsize.
A further object of this invention is to provide a sailboat utilizing multiple-hulls having a high length-to-beam ratio.
Still another object of this invention is to provide a sailboat capable of carrying more sail area per unit weight than known multiple-hull sailboats.
Still another object of this invention is to provide a multiple-hull sailboat capable of carrying more sail area per weight of displacement than known multiple-hull sailboats.
Another object of this invention is to provide a sailboat which will operate with a Bruce Number in excess of 2.0.
A further object of this invention is to provide a multiple-hull sailboat wherein the sail is always inclined upwardly to the windward, to eliminate any downward component of wind force acting on the boat, and wherein the boat hulls operate continuously in contact with the water surface.
Still another object of this invention is to provide a sailboat wherein the boat hulls are mounted relative to the boat frame to pitch independently and thereby accomodate different wave patterns.
A still further object of this invention is to provide a sailboat wherein the presented sail area may be readily changed from 0 to 100 percent.
Another object of this invention is to provide a very light weight sailboat which can safely carry a very large area sail and be operated by a single pilot.
A further object of this invention is to provide a sailboat wherein the frame and mast consist of spars joined by a common coupler enabling different angular relationships and enabling folding for transport or storage.
These objects are accomplished in a sailboat having the following structural features. A rigid frame defines three triangularly spaced support points for support by three respective buoyant hulls, including a windward support point and two lee support points. Three independent buoyant hulls for supporting the frame on the water include a windward hull coupled to the windward support point, and two lee hulls coupled to the two lee support points. Each of the buoyant hulls is elongated and configured to move through the water in either of two opposite directions. Coupling means for each hull includes a vertical pivot axis to enable independent steering of each hull relative to the frame. A mast supported on the frame has a masthead disposed equidistant from the lee support points and toward the windward support point. A sail is supported on said frame generally in a plane defined by the lee support points and the masthead, whereby the sail is inclined upwardly from the lee hulls toward the windward hull.